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What size air purifier do I need? (CADR calculator)

Purifiers are sized by CADR — Clean Air Delivery Rate, in cubic feet per minute. Enter your room; we'll give the CADR to look for on the box, with the math shown.

Presets:

Units

Room area (ft²)

Ceiling height (ft)

Goal

Look for a smoke CADR of at least

333 CFM (566 m³/h)

How the math works

ACH is "air changes per hour" — how many times the purifier moves the room's whole volume through its filter in an hour. EPA and AHAM guidance points to roughly 3–5 ACH; more for smoke. The formula is just volume × changes, converted to minutes:

CADR (CFM) = area × ceiling height × ACH ÷ 60

Cross-check: AHAM's "2/3 rule" says smoke CADR ≥ 2/3 of the room's area in ft² (assuming 8-ft ceilings) — that works out to 5 ACH, the smoke setting above.

Reading the box

Use the smoke CADR (the lowest of the three printed numbers) — it covers the smallest, hardest particles.
An AHAM-Verifide label means the number was independently tested.
Real rooms leak and doors open: buying one size up costs little and lets you run a quieter, lower fan speed for the same clean air.
Open floor plan? Use the area the purifier can actually "see," and expect less at the edges.

What CADR won't fix: CO₂. Filters catch particles, not gases — a stuffy, sleepy room needs ventilation, not more filtration. During smoke events you need both, in turns: the smoke guide explains the sequence.

An air purifier working in a bright living room — The number on its box only means something next to the number for your room.

Questions

Can I use two smaller purifiers?

Yes — CADR adds. Two 150-CFM units ≈ one 300, and they're often quieter combined and cover an odd-shaped space better.

Won't a bigger unit be louder?

Backwards, usually: the oversized unit on its low setting moves the same air as the right-sized one screaming on high — and low settings are the ones people actually leave on.

The box says m³/h, not CFM

Divide by 1.7: 340 m³/h ≈ 200 CFM. (The calculator shows both.)

Sources

US EPA — Air cleaners and air filters in the home (sizing and CADR guidance).
AHAM — AC-1 standard & the 2/3 sizing rule.